1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus of an electrophotographic system like a digital copying machine, a laser printer, a laser plotter, a laser facsimile or a multifunction product including functions of these apparatuses.
2. Description of the Related Art
There is an optical scanning device widely known in relation to image forming apparatuses of the electrophotographic system such as a digital copying machine and a laser printer. In general, the optical scanning device deflects light beams emitted from a light source device with a light deflector, focuses the light beams on a surface to be scanned with an optical scanning optical system, which uses a scanning focus lens like an fθ lens, to form a light spot on the surface to be scanned, and subjects the surface to be scanned to optical scanning (main scanning) using the light spot. The surface to be scanned is essentially a photosensitive surface of a photosensitive medium serving as a photoconductive photosensitive member.
In recent years, color printing using laser printers and digital copying machines has made a great progress. Therefore, an optical scanning device used in these apparatuses is required to form a plurality of scanning lines on a plurality of photosensitive members at a time.
Several methods are conceivable as a system that satisfies such a request. For example, there is a tandem system in which four photosensitive members corresponding to cyan (C), magenta (M), yellow (Y), and black (K) are arranged side by side.
In a color image forming apparatus of the tandem system, the photosensitive members are arranged in a conveying direction of recording paper (or an intermediate transfer member). A light flux of light beams radiated from a plurality of light source devices corresponding to the respective photosensitive members is deflected by one deflecting unit (a light deflector) to scan the photosensitive drums. The respective photosensitive drums are simultaneously exposed to the light flux by a plurality of optical scanning optical systems corresponding to the respective photosensitive members to form latent images. The latent images are visualized by developing devices that use developers of different colors such as cyan (C), magenta (M), yellow (Y), and black (K). The visual images are sequentially transferred onto identical recording paper (or an identical intermediate transfer member) to be superimposed one on top of another. As a result, a color image is obtained.
As an optical scanning optical system for a low-cost optical scanning device suitable for such a color image forming apparatus of the tandem system, there is an oblique incidence optical system. In the oblique incidence optical system, a light beams are made incident on a deflective reflection surface of a light deflector at an angle in a sub-scanning direction with respect to a normal of the deflective reflection surface.
For example, Japanese Patent Application Laid-Open No. 2003-5114 discloses an oblique incidence optical system that makes light beams incident on a deflective reflection surface of a light deflector at an angle in a sub-scanning direction. In an optical scanning device using the oblique incidence optical system, a plurality of light beams made obliquely incident on the deflective reflection surface of the light deflector are deflected and reflected on the deflective reflection surface, respectively. The light beams are separated by a return mirror or the like to be led to surfaces to be scanned (photosensitive members) corresponding to the light beams, respectively. In this case, the angle in the sub-scanning direction of the respective light beams (the angle of oblique incidence on the light deflector) is set to an angle that allows respective light fluxes to be separated by the mirror.
By using the oblique incidence optical system, it is possible to realize intervals of the light beams adjacent to each other in the sub-scanning direction, which allow the respective light fluxes to be separated by the mirror, without increasing a size of the light deflector (e.g., without increasing stages of a polygon mirror in the sub-scanning direction and thickness of the polygon mirror).
However, the oblique incidence optical system has a problem in that a “scanning line curve” is large. An amount of occurrence of the scanning line curve is different depending upon an angle of oblique incidence in the sub-scanning direction of the respective light beams. When latent images drawn by the respective light beams are visualized to be superimposed one on top of another by toners of the respective colors, the amount of occurrence of the scanning line curve appears as color drift. Since the light beams are made obliquely incident on the deflective reflection surface, the light fluxes are made incident on a scanning focus lens in a twisted state. Thus, a wavefront aberration increases. In particular, optical performance is markedly deteriorated at a peripheral image height and a beam spot diameter increases. This prevents improvement of an image quality.
In the oblique incidence optical system, light fluxes from a light source side are made incident toward a rotation axis of the polygon mirror. Thus, when the light source is arranged in a position overlapping an optical axis of the scanning focus lens in a main scanning direction, the angle of oblique incidence is increased to prevent interference with the scanning focus lens.
As a method of correcting the “large scanning line curve” peculiar to the oblique incidence system, a method of including a “lens having a lens surface with an inclination peculiar thereto in a sub-scanning section changed in a main scanning direction to correct a scanning line curve” in a scanning focus optical system is proposed (see, for example, Japanese Patent Application Laid-Open No. H11-14932). A method of including a “correcting reflection surface having a reflection surface with an inclination peculiar thereto in a sub-scanning section changed in a main scanning direction to correct a scanning line curve” in a scanning focus optical system is also proposed (see, for example, Japanese Patent Application Laid-Open No. H11-38348).
In Japanese Patent Application Laid-Open No. 2004-70109, a method of causing light fluxes obliquely made incident to pass outside an axis of a scanning focus lens and aligning positions of scanning lines using a surface for changing a sagittal line of the scanning focus lens along a main scanning direction is proposed. In an example described in the laid-open patent application, correction is performed using one scanning focus lens. Although it is possible to correct the scanning line curve, deterioration in a beam spot diameter due to an increase in wavefront aberration explained below is not described.
Another problem in the oblique incidence system is that large deterioration in a wavefront aberration tends to occur at the peripheral image height (near both ends of scanning lines) because of skew of beams. When such a wavefront aberration occurs, a spot diameter of a light spot increases at the peripheral image height. If it is impossible to solve this problem, it is impossible to realize “optical scanning at high density” strongly demanded recently. In the optical scanning device described in the laid-open patent application, the large scanning line curve peculiar to the oblique incidence system is extremely satisfactorily corrected. However, it cannot be said that correction of the wavefront aberration is sufficient.
As an optical scanning device that can satisfactorily correct the “the scanning line curve and the deterioration in a wavefront aberration”, which are the problems of the oblique incidence system, an optical scanning device in which a plurality of rotational asymmetrical lenses are included in a scanning focus optical system and a meridian line shape connecting sagittal line vertexes of lens surfaces of the rotational asymmetrical lenses is curved in a sub-scanning direction is proposed (see Japanese Patent Application Laid-Open No. H10-73778).
However, the problems are solved by curving the meridian line in the lens having the “lens surface on which the meridian line shape connecting the sagittal line vertexes is curved in the sub-scanning direction”. Separate scanning focus lenses corresponding to incident light fluxes are required. Thus, when the optical scanning device is applied to the tandem scanning optical system, the number of scanning focus lenses increases.
When two light fluxes traveling to different surfaces to be scanned are made incident on an identical lens, the problems are solved for one light flux by curving a sagittal line shape. However, it is difficult to reduce a scanning line curve and a wavefront aberration for the other light flux.
Since the lens has curvature in the sub-scanning direction, when light fluxes made incident on the lens shift in the sub-scanning direction because of influences of an assembly error, a machining error, environmental fluctuation, and the like, a shape of the scanning line curve is affected by an influence of refracting power of the lens in the sub-scanning direction to change. An effect of color drift control in an initial period (or at the time of design) in a color image is not obtained to cause color drift.
Moreover, in the correction of a wavefront aberration, a change in a skew state of light fluxes is large because of fluctuation in incident light fluxes on a surface having curvature. Thus, it is difficult to stably obtain a satisfactory beam spot diameter.
In the conventional technology described in Japanese Patent Application Laid-Open No. 2003-5114 described above, correction of the scanning line curve is carried out using the same surface as that in the conventional technique described in Japanese Patent Application Laid-Open No. H10-73778. However, in this case, it is also difficult to stably obtain a satisfactory beam spot diameter as explained above.
The present invention has been devised in view of the circumstances. It is an object of the present invention to realize an optical scanning device having a new constitution that is suitable for a reduction in cost, a reduction in power consumption, and a reduction in size and can effectively correct a scanning line curve and deterioration in a wavefront aberration in the optical scanning device of the oblique incidence system. It is another object of the present invention to realize an image forming apparatus having a new constitution that effectively corrects a scanning line curve and deterioration in a wavefront aberration in the optical scanning device of the oblique incidence system and has few color drift occurrence even at the time of a high-speed operation, an increase in density, temperature fluctuation, and the like.